The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Aspects of the present invention relate to an assembly for transferring fluids between a vessel and a turret structure mounted in said vessel for a rotation around a substantially vertical turret axis, wherein said turret structure comprises a swivel having at least one fluid path and wherein the fluid path is connected with one end of a fluid piping of which the second end is connected to the vessel.
It is noted that the indication “vessel” also may refer to parts or to a structure fixed to the vessel.
It is known in the offshore industry (and especially the production of fluids such as oil or gas) to use vessels comprising a turret structure mounted in a moon pool for a rotation around a substantially vertical turret axis. The turret structure (or a disconnectable buoy connected to a lower part thereof) may be moored to the seabed by mooring lines and connects risers through which the fluid is delivered to a fluid inlet in the geostationary part of the swivel, which typically is mounted on top of the turret structure or on top of a so-called turntable which is based on and forms part of the turret structure. The fluid is transferred from the rotating part of the swivel to appropriate equipment on board of the vessel through fluid piping connecting the swivel fluid outlet with the vessel (for example a so-called turret access structure, TAS). It is noted that the fluids can be transferred in two directions, both from the risers to the vessel and vice versa. For clarity only the direction towards the vessel is mentioned here, but it should be realised that the indications “inlet” and “outlet” are interchangeable.
The turret structure, including its turntable, the swivel as well as the vessel (for example the TAS) often are subject to considerable loads tending to deform said components. The loads can originate from, among others, gravity, motions, accelerations, vessel deformations, temperature differences, pressure, mooring forces, riser forces, waves, current and wind. Because these components generally have a different stiffness and will be subjected to different loads, their deformations will be different causing relative displacements between these components.
The relative deformations between the rotating part of the swivel and the vessel (or TAS) deform the fluid piping between them, which causes stresses on the swivel and its outlets. These stresses can be low when the piping is flexible and it thus easily conforms to the relative deformations, but can also be very high when the piping is substantially rigid, for example for high pressure piping with a large diameter. These stresses can interfere with the operation of the swivel, for example by damaging it or causing its seals to leak.
Known practice for reducing the mentioned stresses is to increase the flexibility of the fluid piping (for example by allowing multiple bends in the fluid piping with long piping sections between successive bends), together with anchor points for the fluid piping sufficiently far away from the swivel. However, adding such fluid piping lengths is expensive and increases the overall weight acting on top of the vessel or TAS, further increasing the detrimental deformation thereof with resulting increase of the relative deformation between the vessel (or TAS) and the swivel.
Another known practice for reducing these stresses is to increase the stiffness of the components (such as the turret structure and the TAS) in order to reduce their (relative) deformations. This, however, involves the use of extra material (such as steel) with an associated increase of cost and weight.
The effectiveness of the above known stress reduction methods decreases when the structures (TAS, vessel, turret, swivel etc) increase in size. Larger structures will undergo larger relative deformations, while more steel is required to stiffen them and/or more piping bends and lengths are needed to accommodate the relative deformations. This becomes prohibitive in terms of the required amount of constructive material (steel, piping), mass and cost.